Fillion’s memories more specifically pertain to when Whedon spoke about his then-upcoming television show "Firefly" to the enraptured audience of thousands at the annual convention that celebrates and promotes comic books and related popular culture.

Fillion also said he was amazed at Comic-Con in 2012 by the number of people he saw lined up outside the San Diego convention center just to listen to Whedon and former cast members appear on a 10-year anniversary panel of "Firefly." The space western starring Fillion first aired on Fox, but it had a short life and was canceled more than 12 years ago. The show’s episodes were then picked up by the Science Channel.

During that panel, Fillion became emotional about the ending of the show. Whedon also had some feelings to get off his chest about the cancellation of “Firefly,” but said the hard feelings had passed.

"It goes beyond vindication," Whedon said during the 2012 panel discussion. “Vindication came a long time ago. It goes to a place of transcendence.”

Whedon is directing "Avengers 2: Age of Ultron," set to be released May 1, 2015.

As Comic-Con gets underway in San Diego, Nathan Fillion, star of ABC’s “Castle” and who has a cameo in the upcoming “Guardians of the Galaxy” movie, recently told The Hollywood Reporter that he has fond memories of his first Comic-Con, attended with director Joss Whedon.